Mapping the strong interaction between Rydberg excitations in ultracold atomic ensembles onto single photons enables the realization of optical nonlinearities which can modify light on the level of individual photons. This novel approach forms the basis of a growing Rydberg quantum optics toolbox, which already contains photonic logic building-blocks such as single-photon sources, switches, transistors, and conditional pi-phase shifts.

Here, we discuss our recent experiments coupling an optical medium smaller than a single Rydberg blockade volume to a few-photon probe field. Due to the large number of atoms in the blockaded volume and the efficient coupling to the probe light mode, we achieve coherent coupling between the probe field and the effective Rydberg "superatom" even if the probe pulse contains only a few photons. This enables us to study the dynamics of a single two-level system strongly coupled to a quantized propagating light field in free space. 

Furthermore, by controlling the dephasing between the internal degrees of freedom of the superatom, we realize a free-space single-photon absorber, which deterministically absorbs exactly one photon from an input pulse. We show that this system can be used for the subtraction of one photon from the input pulse over a wide range of input photon numbers.